Banks need to build capital buffers when the economy is strong to support it in bad times.
Our Macroprudential Bulletin argues a positive neutral rate of the countercyclical capital buffer and more harmonisation in setting it would help its effectiveness https://www.ecb.europa.eu/pub/financial-stability/macroprudential-bulletin/html/ecb.mpbu202304_01~6eef01bb6a.en.html